In implementing electronic systems or circuits comprising various logic types, it is often necessary to effect logic-level conversions at the interface between circuits of different logic types. A typical TTL-to-CMOS converter (or, alternatively, TTL-to-CMOS Buffer) is shown in FIG. 1. That circuit is used to convert standard TTL output levels (VOL=0.0 to 0.8 volt; VOH=2.0 volts to Vcc) to standard CMOS input levels (VIL=less than 0.3 volt; VIH=greater than (Vdd-0.3 volts)). The switch point for a circuit such as the one set forth in FIG. 1 can be anywhere in the range between 0.8 and 2.0 volts. (For present purposes "switch point" may be understood as the voltage that, when applied at the input, will cause the voltage at the output of the converter to change logic states.) Ideally the voltage will be equal to approximately Vdd/2, where Vdd is the converter supply voltage.
One drawback exhibited by heretofore known logic level converters is the tendency to respond, that is, exhibit a change in output logic state, to spurious or extraneous signals. For example, it has been found that a substantial transient current drain, even though associated with circuitry remote from the converter, can produce momentary changes in the voltage levels of Vdd or Vss. These changes may appear in the vicinity of, or be conducted to, the input circuitry of the logic converter circuit. The converter may then erroneously sense an apparent change in the input voltage. A response by the logic converter to the apparent input transition will cause unintended and invalid data to be propagated through the system.
Therefore, what is desired is a newly configured logic converter, specifically a TTL-to-CMOS Buffer, that exhibits a narrow range of input voltages to which the converter will respond with a change in output logic state. Ideally, the switch point will be midway between VIL, max and VIH, min, or midway between 0.8 and 2.0 volts (=1.4 volts). In addition, the switch point should exhibit substantial stability in the face of variations in operating parameters such as supply voltage, ambient temperature, and so forth.